1. FIELD OF THE INVENTION
The invention relates to a process for the production of a bath of molten metal or alloys wherein liquid nitrogen, argon or carbon dioxide is discharged above the bath of molten metal or alloys throughout the to a related apparatus to discharge said liquid above said bath.
2. PRIOR ART
It is known from British Pat. No. 987 190 to cast continuously a molten metal from a ladle into an ingot mould and to shield the jet of molten metal with a solidified or liquefied inert gas such as liquid nitrogen (when the presence of this element in the metal is not harmful) or argon and to also shield the surface of molten metal in said ladle to avoid oxygen, hydrogen and nitrogen pick-up from the surrounding atmosphere.
In electrical furnaces, molten metal comes from the heating up of pieces of metal or of scrap metal which are progressively melted in said furnace, while new pieces of metal or scrap metal are added throughout the melting phase.
Almost any open face surface of molten metal can be protected against oxygen, hydrogen and/or nitrogen pick-up by injection of liquid argon, nitrogen (if nitrogen pick-up is not a problem) or carbon dioxide snow above the said surface. Said process makes it possible to prevent contamination from atmospheric oxygen and also from humidity generating hydrogen in the melt or from nitrogen in cases where liquid nitrogen is not used.
Furthermore, it is possible with said process to protect the pieces of scrap metal or new stocks of metal in the stage of pre-heating above the liquid bath of molten metal prior to melting. The atmosphere above the metal is selected according to the nature of metals, alloyed metals, alloys or pure metals and it must be maintained above and around the elements of the charge throughout the whole melting and holding operations, from the very moment the charge begins to heat up, to the moment the metal is tapped.
Contrary to the shielding of the surface of molten metal with argon, nitrogen or carbon dioxide in the gaseous state, where the injection velocity of said gases creates turbulence and hence an ingress of atmospheric air diluting the inert atmosphere, protection of the metal with liquefied gases makes it possible for said liquefied gases to reach the bottom of the furnace or the surface of the molten metal : they first vaporize as cold heavy gases (which are heavier then the atmosphere at room temperature) which in turn, heat-up, expand and flush out all the atmospheric air in the furnace.
However, there are some limitations to this protection against hydrogen, nitrogen and/or oxygen pick-ups.
When the pieces of metal are partly covered by water, this water can come into contact with the molten bath and generate hydrogen bubbles in the bath along with some metal oxides. Hydrogen can also be generated by the flames of the burners, if any are used to heat the molten metal. Oxygen can be generated from deeply oxidized scraps of metal introduced in the bath and nitrogen can be generated namely in arc furnaces in the region of electrodes.
As long as liquid argon, nitrogen or carbon dioxide snow is poured onto the surface of the molten bath, air above the surface of said bath is removed, thus, removing oxygen and humidity (water).
However, the very low level of residual oxygen in the vessel, usually below 1%, at the beginning of the process cannot be maintained as soon as the level of molten metal in the furnace reaches about two-thirds of the height of said furnace. Oxygen concentration rapidly increases to reach about 3% to 5% (volume concentration) at this height,which, though still being considered as a good protection, is not completely satisfactory.